Energy Rating for Facades




Paper presented at 'The whole life performance of facades' conference at CWCT, Bath 18 April 2001.


Energy rating assesses the total energy performance of a complete product. Conventional approaches assess how much energy a product loses but take no account of how much energy is gained through solar radiation. Energy rating takes all the energy flows into account and considers the whole of the product (both glass and frame) in assessing how much energy is lost or gained by a specific product. The method clearly identifies the best performing system rather than simply considering individual components.

Energy rating promotes and recognises energy efficient products, provides a clear and unambiguous choice between good products and poor products based on independent testing and approval of the products and acts as a driving force for improved energy efficiency by clearly stating the relative efficiency of each product.

Energy rating makes the adoption of an integrated approach to facade design essential and is an essential tool for improving the whole life performance of facades. Future building legislation throughout the world will undoubtedly reflect increased energy efficiency requirements and energy rating will be an essential legislative tool in these changes.


The vocabulary of the construction industry is rapidly expanding to include new words and phrases such as 'sustainable construction', 'whole life cost', 'embodied energy' and 'life cycle analysis'. The concepts were virtually unknown 20 years ago but are not empty concerns for a planet with finite resources. The energy and environmental performance of a product is rapidly becoming a major indicator of the suitability of the product for use in the construction.

Peter Lyons (Lyons 1998) refers to windows as appliances and says, "Like refrigerators, they are almost always manufactured off-site and then installed, and like refrigerators, they affect the house's total energy use. The way in which the fenestration system (including shades, awnings, and other accessories) is operated also affects the final energy bill for heating, cooling, and lighting." Facades share many of the same features as windows and can in many cases also be considered and treated as 'appliances'. One of the main objectives of this paper is to raise an awareness of this appliance approach to considering the whole life performance of facades.

For most appliances, the cost of energy used over the working life is greater than the capital cost. As an example, the cost of energy used in 40 days of continuous running of any motor is greater than the capital cost of the motor. Similarly the energy cost to run a 'facade appliance' will almost certainly exceed the capital cost of the fašade over the whole life of the fašade. This excludes any concept of 'embodied energy' and is simply the energy used to operate the appliance.

In these terms, energy usage must be regarded as a prime indicator of the whole life performance of the facade.

What is energy rating?

The energy rating of discrete windows is far more developed than the energy rating of facades but many of the same principles apply, the differences between the two applications will be explored later in this paper.

Simply stated, energy rating is a method for assessing the total energy performance of a complete window. 

Conventional approaches (such as setting a glass or window U-value indicator) assess how much energy a window loses but they take no account of how much energy is gained through solar radiation. The concept of a window gaining energy should not be strange to this audience but almost all Building Regulations authorities rate and assess windows simply on the loss side of the equation - the U-value. 

For domestic windows the UK is a heating dominated climate and on a typical domestic heating cycle a well designed and installed window will ideally both minimise the heat losses and maximise the heat gains. For a commercial facade, the heating and occupancy cycles will be different and the ideal facade will minimise heat losses but also control heat gain to prevent overheating of the building during the daytime hours of occupancy. The important point is that the technologies largely exist to predict the needs of the building and the occupants and, more importantly, to specify, design and manufacture a facade to meet these needs.

The total energy flow in a window consists of three major components - the solar heat gain (SHGC) in the form of radiation, the non-solar heat losses and gains from conduction, convection and radiation of all of the components of the window (not simply the glass) and the airflow through the window both designed (ventilation) and unintentional (infiltration). These energy flows are shown in Figure 1.

Energy rating takes all these energy flows into account and considers the whole of the window (both glass and frame) in assessing how much energy is lost or gained by a specific window.

A typical rating method for windows

The British Fenestration Rating Council (BFRC) has produced a Domestic Window Energy Rating Scheme (DWER) to produce a single rating number for window energy efficiency.

The rating number is calculated from the individual values of the major energy transfer mechanisms. These are:
1. The U-value, which measures the overall window U-value and not just the glass component.
2. Solar heat gain coefficient (g), which measures how well a product transmits solar radiation and is a number between 0 and 1. A lower g means less solar heat gain.
3. Air Infiltration (L50), which measures air leakage through the window when it is closed.

These values are applied via the formula:

DWER = (25.9g - 11U - 0.12L50) + 74 (1)

where the correction factor 74 is used to produce a rating number from 1 to 100 for ease of understanding by consumers. A higher DWER rating indicates a more energy efficient window for the UK climate.

A rating system such as this is not an indication of absolute window performance: this will vary with exact size, location, window direction and other factors. The rating is designed to allow accurate comparison of the performance of windows under identical conditions and can be applied to all new and replacement domestic windows in the UK.

Assessment of the values of the individual components of the rating number can be made by either standard hot box testing or by computer simulation. Hot box testing rapidly becomes expensive for manufacturers with a large product range and limits the availability uptake of any system. The availability of low cost finite element computer software provides an economical, rapid and accurate method of assessing the component values for a wide range of products and design variations (Arasteh 1998) and this is the preferred method for the assessment of window properties.

Energy rating systems around the world

Energy rating for windows is an idea being implemented around the world and rating programmes are in place in the USA, Canada and Australia & New Zealand. Many other countries such as the UK (BFRC), Denmark, Russia are either implementing energy rating or in the detailed planning stages.

The American (NFRC) system is based on a statement of the physical properties of the system (U, g and L) and does not attempt to produce a rating number because of the range of climate zones covered by the continental USA (heating dominated, cooling dominated and mixed mode). The Canadian system uses the same physical properties but can combine them into an equation similar to (1) due to the possibility of treating Canada as a single heating dominated climate zone. Individual countries have chosen different approaches to energy rating because of their individual climate conditions but the important point is the emerging groundswell of support for energy rating.

Energy rating is not only for heating dominated climates, in the warmer areas of the USA and Australia the idea of window energy rating is also important, except there the best product is one that rejects solar heat gain and minimises the heat transfer into the building. This reduces the need for air-conditioning (a major energy cost) by keeping the building cooler.

In Europe, similar schemes either starting or being developed. The UK is leading a project to develop a European wide window energy rating system. This is a major EC funded project and will have European wide impact in window selection and design. The climate in the EC is similar to that of the USA and the approach taken will need to reflect the climate variety in the EC. One proposal for climate zones is shown in Figure 2.

Energy rating of facades

An energy rating system for facades must recognise these differences between facades and windows to produce meaningful results. The most important differences are:

1. The purchase/user generally specifies the facade design whereas for windows the purchaser/user selects windows from a specification generated by the manufacturer. Facades have a proliferation of significantly different design options and energy rating needs to be carried out on a contract-by-contract basis. The user can specify the overall energy performance at the design stage whereas this is not possible for windows.

2. Facades are largely site built whereas windows are pre-fabricated and fitted on site. This gives greater control over the build consistency for windows because of the factory finishing but installation control may well be less due to the structure of the industry.

3. The energy cycle for commercial facades is significantly different to that of domestic windows. The occupancy cycle is largely during the day when solar gain is highest and a major concern for facades is significant overheating due to excessive solar gain. This is not a significant concern for domestic windows.

4. Energy rating of facades also has some special concerns with regard to sample selection and definition. Window component sizes are generally broadly comparable and a sample size can be generated which is representative of the application details. Facade support structures can be either thin or thick in aspect. A small sample size will penalise a thick system because of the large amount of frame material present in the sample whereas a large sample size may well penalise a thin system because of the greater number of support members required. This 'sample size' concern has not yet been adequately resolved to define a representative sample size for facade characterisation.

Despite these differences it is possible to construct a meaningful and credible rating system for facades (or site built components) and the USA based NFRC is currently working to produce a 'fair, accurate and credible' system for facade use.

The benefits of energy rating

The buildings sector in Europe accounts for approximately 40% of total energy use, around 70% of which is used in residential buildings (European Commission 1998). The glazing system is a major area of losses in any building and improving the energy efficiency of glazing systems gives a large and nearly permanent improvement to the energy efficiency of a building. Selecting the best system for the application has a direct and quantifiable impact on both energy use and comfort.

Energy rating acts as a driving force for improved energy efficiency by clearly stating the relative efficiency of each product. Users and specifiers can select the most suitable product on the basis of a true cost / benefit analysis. Energy rating clearly identifies the best performing product as a whole system rather than simply considering the frame or the glass.

The cost of fitting high performance products and the payback is such that it is not generally economic to replace products simply to achieve the benefits. If new or replacement windows are being fitted then the extra cost to specify and fit high performance energy rated windows is small and the savings and benefits are significant in both the short and the long term.

Specifiers and building owners

Windows are a major and long-term component of any building. Energy and cost efficiencies secured by using high performance facades is saved for the life of the building and it pays to fit the most appropriate facade for the application.

Energy rated windows produce an enhanced client service by value management of the complete project. They help the client to save money by delivering value for money and can improve client confidence from the use of benchmarks for performance in use. Energy efficient windows reduce whole life costs and improve overall cost in use. Control of energy usage leads to more appropriate (and in many cases cheaper) plant size due to lower energy requirements. Energy efficient windows are cheaper to run than standard windows and even if capital costs are higher these can be traded off against lower running costs.

Energy rated windows can reduce the investment risk and improve the corporate image. For small commercial buildings, energy efficient products improve the working environment and help to increase worker satisfaction and productivity. These factors enhance the value and marketability of the property and improve the rental income.

Energy efficient windows will reduce heating costs for tenants. Tenants prefer energy efficient properties and the use of high performance windows gives many benefits. Satisfied tenants generate fewer complaints, pay rents promptly and leave owners with fewer void properties.


Energy rating has positive benefits for manufacturers irrespective of frame material. Rating promotes and recognises energy efficient products in the market place. The current drive towards claiming "energy efficiency" without any consistent method for measurement or validation of claims only creates confusion in the marketplace and does not allow good manufacturers to prosper.

Energy rating systems provide a clear and unambiguous choice between good products and poor products based on independent testing and approval of the products. It is a new sales aid for manufacturers of energy efficient products and differentiates between competing products and manufacturers. Manufacturers who adopt energy efficiency as a method of distancing their products from price-based competition will see higher performance clearly rewarded with higher ratings.

The current standards

The standards for glazing performance are rapidly becoming harmonised across Europe with work on EN standards either in existence of in the draft stages for most of the basic physical properties (the Level 1 information). These are currently:

U-value (size 1.48 x 1.23):

Radiation properties (glass):

Air permeability (size 1.48 x 1.23):

There are some differences between the ISO approach to several of the Level 1 properties and the EN approach but these are predicted to converge in the near future as the technology advances. The essential point is that the methodology for the determination of the basic Level 1 information is rapidly becoming well defined. This then allows the information to be assembled into the energy rating number (the Level 2 information) as is required by the user and the rating system chosen.

The CE marking of products presents both an opportunity and a concern. The CE mark requirements must be met and provide a framework for information transfer but equally it is difficult to produce and label information which 'conflicts' with the CE requirements. Certification in the future will be interesting.

The legislative framework

The legislative framework for energy rating is a local activity and varies widely throughout the world. The European area is seeing a constant increase in the requirements for energy efficiency of buildings but this is mainly based on the concept of decreasing the allowable U-value for the glass. As yet there is little evidence of moves to consider the whole product and the effects of solar heat gain. This is primarily due to the lack of established energy rating systems to provide the regulatory authorities with benchmarks for specifying performance. In the USA, local code authorities are increasingly using the published NFRC ratings as a specification method for building requirements and NFRC ratings are now adopted in a majority of code areas.

The availability of rating systems information will undoubtedly increase the specification of products by the results of the rating system and this is already being forecast in the proposed revisions to the Building Regulations in the UK where energy rating has been suggested as a possible specification method for signifying compliance with the Building Regulations.

Energy rating will become invaluable in providing proof of conformance with increasing energy efficiency requirements of the building regulations and energy rating will clearly identify and reward manufacturers of energy efficient windows. Future building legislation throughout the world will undoubtedly reflect increased energy efficiency requirements and energy rating is an essential legislative tool in these changes.

The design tools

Energy efficiency is not simply dependent on the glass or frame material used. It is possible to make a thermally efficient window from almost any conventional frame material. Some materials are less thermally efficient than others but good design and manufacture can produce high performance windows from any of the common framing materials. Window energy rating clearly identifies the best performing window as a whole system rather than simply considering the frame or glass.

Energy rating makes the adoption of an integrated approach to facade design essential. Energy efficiency cannot be considered to be a 'bolt-on' extra but must be designed into the product. Unlike hot box testing, which requires physical samples, the software tools available can be used at the design stage. This allows the use of preliminary CAD drawings to assess the eventual performance of the facade and to provide an energy rating estimation before the product design is complete.

In future, the assessment of the energy aspects of whole life performance of the facade will begin at the design office.

The future

Facades have a long life cycle and the energy saved by fitting high performance facades is effectively permanent in the effects - simple decisions and small incremental costs at the ordering and specification stages can significantly reduce capital costs and annual fuel bills, improve comfort and reduce greenhouse gas emissions for the life of the installation. 

Energy rating provides the designer and specifier with the only effective framework and set of tools for making informed decisions on the costs and benefits of improved energy efficiency.

Energy rating is an essential tool for improving the whole life performance of facades.


 Last edited: 11/03/10

© Tangram Technology Ltd. 2001

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